The present provisional application relates to the field of magnetic resonance imaging or "MRI".
MRI is widely used in medical and other arts to obtain images of a subject such as a medical patient. The patient's body is placed within the subject-receiving space of a primary field magnet and exposed to a strong, substantially constant primary magnetic field, so that the nuclei spin around axes aligned with the magnetic field. Powerful radio frequency ("RF") signals are broadcast into the subject receiving space to excite atomic nuclei within the patient's body into nuclear magnetic resonance. The spinning nuclei generate minuscule RF signals, referred to herein as magnetic resonance signals. By applying magnetic field gradients so that the magnitude of the magnetic field varies with location inside the subject-receiving space, the magnetic resonance phenomenon can be limited to only a particular region or "slice" over the patient's body, so that all of the magnetic resonance signals come from that slice. Moreover, by applying such magnetic field gradients, characteristics of the magnetic resonance signals from different locations within the slice, such as the frequency and phase of the signals can be made to vary in a predictable manner depending upon position within the slice. Stated another way, the magnetic resonance signals are "spatially encoded" so that it is possible to distinguish between signals from different parts of a slice. After performing many excitations under different gradients, it is possible to derive a map showing the intensity or other characteristics of magnetic resonance signals versus position within the slice. Because these characteristics vary with the concentration of different chemical substances and other chemical characteristics of the tissue, different tissues provide different magnetic resonance signal characteristics. When the map of magnetic resonance signal characteristics is displayed in a visual format, such as on a computer screen or printed image the map forms a picture of the structures within the patient's body, with different tissues having different intensities or colors.
The RF excitation signals are normally applied by antennas fixed to the primary field magnet structure and arranged to provide substantially uniform excitation throughout the subject-receiving space. The RF excitation signals are provided by powerful radio transmitters. The magnetic resonance signals, which are many millions of times weaker than the RF excitation signals, can be received by antennas mounted on the primary field magnet or, more commonly, by antennas placed close to the area of the patient's body to be imaged.
A difficulty encountered with conventional RF transmitting antennas mounted on the primary field magnet relates to the electrical interaction between the antenna and the remaining structure of the magnet. Typically, RF transmitting antennas have been provided as coils arranged in a plane, with the plane of the coil closely overlying an electrically conductive part of the magnet structure, most typically the pole piece of the magnet. To conserve room within the subject receiving space and leave a large open area for the patient, it is desirable to place the coil as close as possible to the magnet structure. However, the transmitting antenna and the magnet structure cooperatively act as a capacitor. When the transmitting antenna is arranged in close proximity to the magnet structure, a so-called "parasitic capacitance" is introduced into the electrical circuit of the transmitting antenna. This, in turn, causes problems in tuning the antenna. To provide efficient RF signal propagation, the resonant frequency of the transmitting antenna circuit must be equal to the frequency of the RF excitation signals to be sent and hence, must be equal to the resonant frequency of the atomic nuclei. The resonant frequency of the antenna circuit is inversely related to the inductance and the capacitance present in the circuit as a whole. The antenna has electromagnetic inductance. Preferably, the parasitic capacitance of the antenna together with the inductance of the antenna provide an untuned resonant frequency higher than the desired resonant frequency to match the RF excitation frequency. It is a simple matter to connect an additional capacitor into the transmitting antenna circuit so as to reduce its resonant frequency and thereby match the resonant frequency of the antenna circuit to the RF excitation frequency. However, where the parasitic capacitance and the natural inductance of the antenna, without any added capacitance, yield a resonant frequency below the RF excitation frequency, the antenna circuit cannot be tuned to the RF excitation frequency.
Accordingly, it is desirable to minimize the parasitic capacitance of the antenna structure. This need is especially apparent where the apparatus is to operate at relatively high magnetic fields, such as at about 3 kilogauss or about 6 kilogauss, and hence operates at high radio frequencies, such as about 12 to about 25 megahertz or higher. The copending, commonly assigned U.S. Patent Application of Charles Green et al. entitled Magnetic Resonance Imaging Excitation and Reception Methods and Apparatus, Ser. No. 08/683,623, filed Jul. 17, 1996 (the "Green et al. application") discloses an effective solution to this problem. The disclosure of said Green et al. application is hereby incorporated by reference herein. One of the teachings in the Green et al. application discloses the use of principal RF antennas mounted to the magnet structure. The principal RF antennas have windings defining coil surfaces substantially transverse and preferably perpendicular to the adjacent conductive surfaces of the primary field magnet. As used herein, the term "coil surface" refers to an imaginary surface defined by the central axes of the conductors constituting a coil or antenna. For example, in the particular case of a flat, loop-like coil lying in a plane, the coil surface is the plane of the coil. Other, more complex coil shapes may define curved coil surfaces. Thus, as used herein the term "coil surface" has the same meaning as in the Green et al. application. The Green et al. application also discloses the use of a local retransmitting antenna separate from the antenna for concentrating the RF energy applied by the principal antenna into the small region within the subject receiving space of the apparatus encompassing the region of interest within the patient or other subject to be scanned. For example, in some embodiments of the Green et al. application, the local retransmitting antenna may include a coil encircling a portion of the patient's body to be imaged.
Nonetheless, still further improvements and alternatives would be desirable. As more fully set forth in the Appendix below, parasitic capacitance between the windings of a coil and a conductive surface of the MRI apparatus is electrically equivalent to a capacitance in series with the coil. For a given physical configuration of the coil winding and conductive surface, having a given parasitic or stray capacitance per unit length of the winding or coil conductor, the effective capacitance will have one value ("C") where the coil has an end connected to the conductive surface as, for example, in the common case where one end of the coil is connected to ground and the conductive surface is also connected to ground. However, if the same coil, having the same parasitic capacitance per unit length is isolated from the conductive surface so that the coil is not conductively connected to the conductive surface, the equivalent capacitance will be reduced substantially to a value of C/4. That is, the effect of parasitic or stray capacitance is reduced by approximately 3/4 where the coil is isolated from the conductive surface.
According to the present invention, a composite antenna structure includes an isolated coil and an active coil. The isolated coil is inductively linked to an active coil. Typically, tuning capacitances are connected to each of the coils. Typically, one end of the active coil is connected to ground, and the conductive surface of the MRI magnet apparatus is also connected to ground. The isolated coil, with its connected tuning capacitance, forms a closed loop circuit. The isolated coil is not conductively coupled to ground. Where the composite antenna is used as a transmitting antenna for sending RF excitation into the subject receiving space, the active coil is connected to the RF signal output of the RF transmitter. Conversely, the composite antenna can be used as a receiving antenna in which case the other end of the active coil is connected to the signal input of a receiver.
In general, the degree of electromagnetic coupling between an antenna structure and an object such as a patient disposed within the subject receiving space increases as the number of windings in the antenna structure increases. In the preferred antenna structures according to the present invention, some of the windings are provided in the form of isolated coils. In these preferred composite structures, the effect of stray capacitance is substantially lower than in a comparable antenna structure in which some windings are placed geometrically at the same location, but all of the windings are connected as part of a single, active coil.